The US president, Barack Obama, in The Hague at the nuclear security summit. Photograph: Robin Van Lonkhuijsen/AFP/Getty Images

President Barack Obama has described Russia as no more than a "regional power" whose actions in Ukraine are an expression of weakness rather than strength, as he restated the threat from the G7 western allies and Japan that they would inflict much broader sanctions if Vladimir Putin went beyond annexation of Crimea and moved troops into eastern Ukraine.

Speaking at the end of a summit on nuclear security in The Hague, Obama rejected the suggestion made by Mitt Romney – his Republican challenger in the last president election – that Russia was the United States' principal geopolitical foe. The president said he was considerably more concerned about the threat of a terrorist nuclear bomb attack on New York.

He said that the US was committed to the defence of its Nato allies but that for non-member states along Russia's borders, Washington and the rest of the international community would use non-military pressure to counter Russian encroachment.

Obama said he would not guess at Putin's motivation, but his remarks appeared to confront one of the apparent aims behind the Russian leader's actions so far in Crimea – to restore the superpower status and prestige Moscow once enjoyed as the capital of the Soviet Union.

"Russia is a regional power that is threatening some of its immediate neighbours, not out of strength but out of weakness," the president said. The US also has influence over its neighbours, he added, but: "We generally don't need to invade them in order to have a strong cooperative relationship with them.

"The fact that Russia felt it had to go in militarily and lay bare these violations of international law indicates less influence, not more," Obama said.

"Russian actions are a problem. They don't pose the number one security threat to the United States. I remain much more concerned about the prospect of a nuclear weapon going off in Manhattan," the president said, pointing out that was why he had begun a series of bi-annual summits aimed at improving the security of weapons-grade fissile material stocks around the world.

At this week's summit in The Hague, the third in the series, 35 countries agreed to abide by a set of international standards governing security at nuclear sites and accept international inspections of their security arrangements. However, Russia and China were among the 18 countries at the summit that did not sign the agreement, a reflection of how old Cold War era divisions remained, and have deepened with the Ukraine crisis. China has remained neutral over Russia's annexation of Crimea, abstaining in a UN Security Council vote and refusing to respond to western invitations to join international condemnation of the move.

In his remarks in The Hague, Obama said that he did not think that the Russian leadership, which has massed troops on Ukraine's eastern border, had made a decision yet over whether to invade that part of the country.

"If they stay on Russian soil, they pose what appears to be an effort of intimidation, but Russia has the right legally to have its troops on its own soil. I don't think it's a done deal," the president said. He added that he thought Moscow was "making a series of calculations" and that those calculations would be affected by how the US and the international community respond.

He adamantly rejected Russian justification for the invasion and annexation of Crimea on the grounds that Russian speakers were under threat, and rejected Moscow's comparisons to Kosovo, whose declaration of independence – a decade after the start of a Serbian campaign of ethnic cleansing of its ethnic-Albanian majority – was recognised by western capitals.

"There has been no evidence that Russian speakers have been in any way threatened," Obama said. "When I hear analogies to Kosovo, where you had thousands of people who were being slaughtered by their government, it's a comparison that makes absolutely no sense." "I think it is important for everybody to be clear and strip away some of the possible excuses for potential Russian action."

The president acknowledged that the broad sanctions on the energy, finance, and arms-export sectors of the Russian economy, threatened by the G7 in the event of further territorial expansion by Moscow, would also have an impact on the West, but he vowed that the impact on Russia would be far worst.